Issue Archive

One glance under the hood of a modern automobile is all it takes to realize that
free space in the engine compartment is a thing of the past.

If carmakers could reduce the number,
size, and weight of the components
in there, better fuel economy
would result. A case in point is the
design and development of optimized
cooling structures, or advanced heat
sinks, for thermally regulating the growing
number of power electronics components
used in the electrical system of
Toyota hybrid vehicles.

To save the time and expense associated
with analytical design methods and
trial-and-error physical prototyping, researchers at the Toyota Research Institute
of North America (TRI-NA) in Ann
Arbor, MI, instead used numerical simulation
and multiphysics topology optimization
techniques to design, fabricate, and
test possible prototypes of a novel heat
sink for future hybrid vehicle generations.

One example prototype combines single-
phase jet impingement cooling in
the plate’s center region with integral
hierarchical branching cooling channels
to cool the periphery. The channels
radiate from the device’s center where a
single jet impinges, and carry liquid
coolant across the plate to dissipate heat
evenly throughout and with minimal
pressure loss.

Numerical simulations enabled Dr.
Ercan (Eric) Dede, Principal Scientist in
TRI-NA’s Electronics Research Department, and colleagues to produce the
optimized branching cooling channel patterns
in an automated fashion using
advanced simulation tools as opposed to a
traditional trial-and-error design approach. He carried out this work as part of
TRI-NA’s mission to conduct accelerated
advanced research in the areas of energy
and environment, safety, and mobility
infrastructure. TRI-NA is a division of the
Toyota Technical Center, which in turn is
part of Toyota Motor Engineering &
Manufacturing North America, overseeing
R&D, engineering design and development,
and manufacturing activities for
Toyota’s North American plants.

TRI-NA’s Electronics Research Department focuses on two main areas:
sensors and actuators, and power electronics.
Among its resources are powerful
modeling and simulation capabilities
and prototype design tools, which
enable its staff to develop effective solutions
in the compressed timeframes
demanded by the highly competitive
automotive markets.

Hot Under the Hood

Toyota hybrid vehicles have sophisticated
electrical systems in which many
power diodes and power semiconductors
such as insulated gate bipolar transistors
(IGBTs) are used for power conversion
and other applications. These components are standard planar silicon
devices measuring a few centimeters per
side, with high power dissipation.

In these hybrid vehicles, they are
mounted on aluminum heat sinks, or
cold plates, through which a water/glycol
coolant mixture is pumped. In earlier
model years, the cold plate design featured
a fluid inlet on one side of the
plate, an outlet on the other side, and in
between were arrangements of mostly
straight cooling channels through which
the coolant flowed. The long channels
provided adequate heat transfer but it
came at the cost of a significant pressure
drop across the plate.

However, the technology roadmap for
these power components calls for them
to shrink to about half their current size
while dissipating the same amount of
power, meaning that heat fluxes will
have to increase. In addition, although
they have a 150 °C maximum operating
temperature, typical silicon devices are
kept at lower temperatures for greater
component reliability. Moreover, the
role of such devices is becoming more
important as the electrification of vehicle
systems increases.

All of these factors mean that thermal
management of these devices will
become more difficult than it has been
to date. It might seem reasonable to simply
redesign the cold plates so that more
coolant can be pumped through them.
But that would require more pumping
power, and with space already at a premium
in the engine compartment
where the pump is located, moving to a
larger, more powerful pump or adding
an additional pump is unacceptable.

Instead, Toyota decided to look at
reengineering the cold plate with an eye
toward achieving optimum heat transfer
and negligible additional pressure drop
simultaneously. If both could be
achieved, thermal objectives could be
met at no significant increase in system
pumping capacity.

Question of the Week

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